A typical mobile communication system covers a large geographical area and comprises a plurality of cells, each of the cells being generally served by one base station. The cells vary greatly in size, usually depending on the call traffic in the area. Cell size is generally smaller in areas with high traffic density than in regions where fewer calls are established. Regardless of the cell size, on many occasions it would be useful to know the geographical location or direction of travel of a mobile station.
Location information on a mobile station can be used for many purposes. If desired, the user can determine his or her own location, call charging can be location-dependent or the location of a mobile station placing an emergency call can be determined so that help could be sent. For instance, vehicles may carry an arrangement for an automatic emergency call in case of an accident, the emergency call also comprising information on the vehicle's location.
Several methods have been developed for measuring the location of mobile stations. For instance, the distance of a mobile station from a base station with which the mobile station communicates can be measured on the basis of a signal propagation delay. However, it is not relevant to the operation of the present invention how the location data is measured, and therefore, as regards location measurement, reference is made to GSM specification (GSM 02.71) and to WO 98/16078, for instance. It is obvious to the person skilled in the art that location measurement can also be carried out by employing other methods than those disclosed in said publications.
The above location measurement methods have a common feature that if post-measurement location determination, i.e. interpretation of the measurement results, employs only the last measurement, the accuracy of the determined location equals that of the measurement concerned. Inaccuracy in a location determined by one-point method can be several hundreds of meters, depending on the location measurement method. Consequently, when the mobile station moves in the vicinity of an oblong natural obstacle, it may be difficult to determine on which side of the obstacle the mobile station is located.
Hence, when the mobile station is located on one side of an oblong natural obstacle, such as a river, lake, park, hill or railway, it may happen that due to inaccuracy in the location measurement method it is not known, in case of traffic accident, on which side of the natural obstacle the emergency vehicle should be sent.